As a porous fiber, an acrylic porous fiber having voids formed during wet; spinning and a polyester-based porous fiber obtained by adding an elutable component, melt-spinning a fiber and then alkali-eluting the component are known, and there is further known a polyolefin-based hollow porous fiber having slit-like pores, produced by melt-spinning a polyolefin under high draft to obtain a hollow type fiber, heat-treating it to promote its crystallization and stretching it at a plurality of stages, as is described in Japanese Patent Publication No. 52123/1981.
However, the above acrylic porous fiber and polyester-based porous fiber, which are fibers having such a size as to be normally processable with a fiber machine such as a carding machine, have problems in that their void percentage, specific surface area, fiber surface opening ratio and pore diameter are all small due to their characteristics derived from the production process thereof and that, even if they are used as an adsorption material or a reserve substrate, they are therefore insufficient in adsorption amount and liquid retention amount and show a low adsorption rate and a low liquid absorption rate. Further, in terms of quality, they are corroded with an organic solvent and a strong alkali.
On the other hand, as far as the polyolefin-based hollow porous fiber is concerned, the chemical resistance is almost no problem, whereas it is difficult to obtain a fiber having a size of 50 denier or less which can be processed with a general fiber machine, and no polyolefin-based hollow porous fiber is commercially available. A fiber having a large size can be prepared into a fabric-like form only when its continuous filament is woven into a cloth. Meanwhile, an adsorbent having fine interstices like those of a nonwoven fabric is suitable as an adsorbent. However, a fiber having a large size can be formed only into a cloth, and a substance to be adsorbed passes through the cloth. Therefore, its adsorption efficiency is poor, and the adsorption performance which the fiber inherently has cannot be effectively used.
Further, in the porous fiber obtained by the production process described in Japanese Patent Publication No. 52123/1981, slender slit-like pores are dispersed on the fiber surface, and the pores are characteristically extending from the fiber surface toward its center nearly linearly in the cross section. For this reason, for example, even if an attempt is made to make a particulate substance adsorbed or sealed in, the above porous fiber cannot cope with particles having a larger particle diameter than the slit width. That is, in a substantial sense, the above porous fiber can be used only for adsorbing fine particles having a size of 0.1 .mu.m or less.
Meanwhile, an attempt is being made to impart a fiber product with a deodorant function, and a number of processing methods therefor have been proposed. As typical examples of the processing methods of this type, there are known a method in which a porous substance having adsorption performance such as zeolite, activated carbon, silica gel, etc., is mixed with a synthetic resin material, the mixture is melt-spun to form a fiber and the fiber is stretched to expose the porous substance at the surface of the fiber, and a method in which a deodorant substance such as a porous substance is coated on the surface of a synthetic fiber produced by a general method. However, in the above method For producing a deodorant fiber by mixing a porous substance, the amount of the porous substance is increased to improve the deodorant function of the fiber. When the amount of the porous substance is increased, however, the fiber formability and stretchability decrease at the production step. As a result, the fiber that can be obtained is nothing but a fiber having a relatively large diameter. Moreover, since the internally embedded porous substance does not much contribute to the improvement in the deodorizing performance, the resultant fiber has too low deodorant performance for the amount of the added porous substance.
On the other hand, in the method in which the synthetic resin fiber is surface-coated with a deodorant substance, the improvement that can be expected in the deodorant performance is limited since the fiber surface area is limited. Further, there is another problem in that the deodorizing performance decreases since the deodorant substance drops off during the processing step.
The present invention has been made in view of these conventional problems. The object thereof is to provide a polyolefin porous fiber having such excellent chemical resistance as to be able to cope with a variety of substances to be adsorbed, having a large specific surface area and a large pore percentage, having a large surface opening ratio, and being processable with a general fiber machine, and a process for the production thereof.